1. Field of the Invention
The present invention generally relates to an ATM (Asynchronous Transfer Mode) network, and more particularly to an accounting system and method suitable for an ATM network.
2. Description of the Related Art
Recently, there has been considerable activity in the development of an ATM network suitable for constructing a B-ISDN (Broad-band Integrated Services Digital Network). In such an ATM network, information with a fixed length (53 octets) is transferred. Such information having a fixed length is called a cell. The ATM network is capable of transferring various types of information (media). Examples of the media transferred are digitized speech signals having low bit rates, and data having high bit rates, such as image data, facsimile data and still picture data. It is necessary to charge users for communications services provided by the ATM network.
The ATM network has not yet been practically used, and attractive accounting systems have not yet been proposed. One may consider an accounting system based on the quantity of data transferred via the ATM network.
FIG. 1 shows a part of a switch in a conventional ATM network equipped with the above accounting system. The switch shown in FIG. 1 comprises a buffer 12, a signaling device 13, a header conversion circuit 14, a cell counting circuit 15, an accounting data memory 16, a call control device 17, and a conversion table 18. A call setup procedure is carried out between calling and called terminals (not shown) and the signaling device 13. The call control device 17 determines a route directed to a destination at which the call will terminate. Cells transmitted by the calling terminal after the route is determined are successively stored in the buffer 12.
FIG. 1 shows one input cell having information and a header 10. The header 10 contains a VCI (Virtual Channel Identifier) assigned to the subscriber line (input or incoming line) via which the above call is transferred. The VCI of the input cell shown in FIG. 1 has a value of "a". The header conversion circuit 14 detects the VCI value of the input cell. Regarding the call from the terminal, the call control device 17 has assigned another VCI value to the route directed to the destination determined in the above-mentioned manner, and has written the above VCI value into the conversion table 18 provided in the header conversion circuit 14. In the example shown in FIG. 1, a VCI value ".beta." has been assigned to the route and has been written into the conversion table 18.
The input cell is written into the buffer 12 and the header 10 of the input cell is copied and written into the header conversion circuit 14. The header conversion circuit 14 detects the VCI value (=a), and obtains the VCI value (=.beta.) assigned to the route by referring to the conversion table 18. In the example shown in FIG. 1, the header conversion circuit 14 accesses the conversion table 18 by the VCI value "a", and obtains the VCI value ".beta." assigned to the route directed to the destination. Then, the header conversion circuit 14 writes the VCI value ".beta." in the buffer 12.
The cell with the VCI value ".beta." is read from the buffer 12 and output to an output highway (outgoing line). The header conversion circuit 14 outputs a signal to the cell counting circuit 15 each time it receives the cell. The cell counting circuit 15 increases the count value for each VCI value each time the signal from the header conversion circuit 14 is received. The count value of the cell counting circuit 15 obtained for each VCI value is written into the accounting data memory 16. The charge for each VCI value can be calculated using the respective count value.
As has been described previously, various types of information are transferred via the ATM network. For example, a speech signal has a bit rate of, for example, 64 Kbps and an image signal has a bit rate of, for example, 100 Mbps. In this case, the number of cells containing image data is approximately 1000 times the number of cells containing speech data. According to the above-mentioned accounting system, the charging of money is carried out for each VCI value. Since the charging rate is determined based on one cell, in the above case, the charge for transfer of the image signal is approximately 1000 times that for transfer of the speech signal. However, this is not reasonable and practical. If the charging rate is determined based on a cell for the transfer of speech, the charge for transfer of image data is extremely expensive. On the other hand, if the charging rate is determined based on a cell for transfer of image data, the charge for transfer of speech data is extremely cheap.